1. Field of the Invention
The present invention relates to a head lamp device for a vehicle.
2. Description of the Related Art
A head lamp for a vehicle generally comprises a reflector in the shape a paraboloid of revolution provided on the inner surface of a rear wall of a lamp housing; a light bulb provided near the focus of the reflector; and a light source front lens covering a front opening of the lamp housing. Light from the light source is reflected by the reflector to form beams nearly parallel to the optical axis. These beams are diffused by the front lens to illuminate the road ahead with a predetermined luminous intensity distribution pattern (e.g., Japanese Laid-Open Patent Publication No. 2-12702).
The head lamp of a vehicle has usually two states, i.e., a driving beam for forward and upward illumination, and a passing beam for forward and downward illumination. The driving beam is; used when the vehicle runs on the road without any cars running on the opposite lane. Whereas the passing beam is used either while a car is running on the opposite lane, or during a drive in a bright urban district. According to the Safety Standard, the driving beam and the passing beam are required to provide a predetermined visible range, respectively. Further, the head lamp should permit light to be dimmed, or the axis of projected light to be directed obliquely downward, so as not to interfere with driving of other vehicles.
The head lamp in customary use is broadly classified into a two-lamp system having a right lamp unit and a left lamp unit on the front of the vehicle, and a four-lamp system having two right lamp units and two left lamp units on the front of the vehicle. With the two-lamp system, one lamp unit contains a light bulb which provides both a filament for driving beam and a filament for passing beam. With a certain type of the four lamp system, a driving beam lamp unit having a light bulb which provides a single filament for driving beam and a passing beam lamp unit having a light bulb which provides a single filament for passing beam are provided in pairs, and the two pairs are mounted on both sides of the front of the vehicle. In both of the above, only the filament for passing beam is lit when projecting the passing beam, while the filaments for passing beam and driving beam are both lit to project the driving beam.
In recent years, a discharge lamp has been utilized as a light source of a head lamp. Compared with a conventional filament type lamp, the discharge lamp radiates light of high luminance with markedly reduced electric power and remarkably long life. Thus, it is highly effective for improving the forward visibility and for energy saving.
However the discharge lamps need to be supplied with relatively high voltage of electricity, and therefore a lighting circuit for raising the voltage of electricity to be supplied to the discharge lamps is required, and it is difficult for the lighting circuit to be shared by a plurality of discharge lamps. Hence, a total of four lighting circuits (two on each side of the vehicle) are needed, costing very high.
To make up for this disadvantage, an idea has been to use a discharge lamp as a light source on the passing beam side, and use a conventional inexpensive light source, such as a halogen lamp, for the driving beam side. With such a system, the discharge lamp and the conventional light source are different in color temperature when lit. Light from the driving beam side looks relatively dimmer than light from the passing beam side. The luminous intensity distribution is also ill-balanced between the driving beam side and the passing beam side.